This question should be the one that needs to be addressed by linguistic researches or studies so let's start by admitting that what I write here probably won't be very meaningful. There are two distinct ways to teach foreign languages. One way should be started as soon as possible like babyhood and this isn't effective after a certain age. One will learn the language like a mother tongue via this way, but some claim that this might hamper his/her development of language ability because more than one language mingle. The other way suggests to introduce the foreign language after one gets solid structure of his/her mother tongue. In this case, one usually needs to start with the grammar and be educated in his/her mother tongue at least for some time save for some people gifted with language. = at least they would be able to save time because they're already like that.  It is controversial whether or not one who learned a foreign language through this way can eventually speak the foreign language as if he/she is a native. I can't actually judge the superiority between two methods since I strictly followed the second one.